1. Field of the Invention
The present invention relates to a high-frequency filter, a multiplexer, a high-frequency front-end circuit, and a communication device each including an elastic wave resonator.
2. Description of the Related Art
To date, a high-frequency filter having an elastic wave resonator has been proposed. For example, a high-frequency filter including an LC resonant circuit and an elastic wave resonator has been disclosed (see, for example, Japanese Unexamined Patent Application Publication No. 2009-538005). In this high-frequency filter, the elastic wave resonator and the LC resonant circuit each define a band-elimination filter (BEF). This configuration makes it possible to form two stop bands: a stop band of the elastic wave resonator near a pass band and a stop band of the LC resonant circuit that is higher than the stop band of the elastic wave resonator by one octave or more.
Although the existing high-frequency filter described above is able to increase the steepness of an attenuation slope formed to extend from the pass band to the stop band with a sharp attenuation pole of the elastic wave resonator, the stop band of the elastic wave resonator is narrow. For example, recently, a multiplexer in which respective terminals at ends of a plurality of high-frequency filters are guided into a common terminal has been used to handle carrier aggregation (CA). Such a multiplexer is designed such that the stop band of one of the plurality of high-frequency filters overlaps the pass band of another one of the high-frequency filters. However, in a case in which the stop band of the one high-frequency filter is narrow, it is difficult to sufficiently attenuate a signal in the pass band of the other high-frequency filter. In this respect, for example, increasing the number of elastic wave resonators included in the high-frequency filter makes it possible to broaden the stop band. However, an insertion loss within the pass band increases as the number of elastic wave resonators is increased.